You can just use string.Contains to see if the agent has 'Blackberry' within it
Something like..
string agent = Request.UserAgent;
if (agent.ToLower().Contains("blackberry"))
{
}
This should tell you that the client is using a blackberry device, but nothing more specific than that. Is that what you wanted?
Edit after Comment:
Possibly look into the 51degrees mobile device detection project which uses the WURFL device database
http://51degrees.codeplex.com/releases/view/50532[
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http://51degrees.codeplex.com/documentation[
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This should allow you to determine device models without having to add lots of string parsing